To reduce cost and a construction period without enlarging the width of a lower half cross section although the width of an upper half cross section of a tunnel is enlarged.

An upper half cross section 16 is excavated to enlarge width along the insert direction of a pipe 12. Upper half timbering 20 is installed at the excavated upper half cross section part, and ribs 22a-22e are provided on the inside of the lower end part of the upper half timbering 20 installed at the upper half cross section 16. A lower half cross section 17 of a tunnel 10 is then excavated without width enlargement. Lower half timbering 21 is installed at the lower half cross section 17, and the ribs and the lower half timbering 21 are connected to form lining 25 for burying the respective timbering 20, 21 over the whole cross section of the tunnel 10.